1. Field of the Invention
The present invention relates to an angular velocity sensor, and more particularly to a micromachined angular velocity sensor, which can be used for a hand shake compensating camera, a car navigation device or the like.
2. Description of the Related Art
As a conventional angular velocity sensor using a micromachining technique, an angular velocity sensor 80 disclosed in Japanese Unexamined Patent Publication No. 6-174739 is described referring to FIGS. 18 and 19. A reference numeral 81 denotes a frame formed by machining a silicon substrate, and beams 82a-82d respectively extending orthogonal to an inner wall of the frame 81 are coupled with each other at a center part of the frame 81. A vibrating weight 83 is formed on a lower part of this center part. These beams 82a-82d and the vibrating weight 83 are integrally formed with each other through the same silicon substrate as the frame 81 using semi-conductor micromachining technology such as photo-etching.
Piezoelectric elements 84a, 84c for drive are formed on an upper surface side of the beams 82a, 82c opposite to each other, while piezoelectric elements 84b, 84d for detection are formed on an upper surface side of the beams 82b, 82d opposite to each other. The piezoelectric elements 84a, 84b for drive and the piezoelectric elements 84b, 84d for detection are of the structure in which a zinc oxide film 87 is interposed between an upper electrode 86a and a lower electrode 86b. 
When drive signals which differ in phase by 180xc2x0 are respectively added to piezoelectric elements 84a and 84c for drive, the beams 82a and 82c vibrate in the vertical direction with a base point 85 as a node as illustrated by the broken line and the two-dot-chain line due to the reversed phase, and a lower tip of the vibrating weight 83 vibrates in the X-axis direction.
When the rotation is thus added about the Z-axis passing through the center of the vibrating weight 83 when the vibrating weight 83 vibrates, the lower tip of the vibrating weight 83 is also vibrated in the Y-axis direction due to the Coriolis force. This vibration is detected as the voltage by the piezoelectric elements 84b, 84d for detection, and the angular velocity of the rotation is obtained by achieving the differential amplification thereof.
In this conventional angular velocity sensor 80, however, internal stress is left behind in the beams 82a-82d due to shrinkage during the crystallization of zinc oxide when the zinc oxide film is formed on silicon to constitute the beams 82a-82d . When the excitation frequency is changed, the excitation amplitude shows a hysteresis characteristic and the excitation vibration becomes unstable.
Further, in the conventional angular velocity sensor 80, the stress in the beams 82a-82d is changed by temperature due to the difference in the coefficient of thermal expansion between silicon and zinc oxide which constitute the beams 82a-82d . The resonance frequencies of the beams 82a, 82c for drive and the beams 82b, 82d for detection are separately changed, and the temperature drift of the angular velocity detection sensitivity is increased thereby.
Also, in the conventional angular velocity sensor 80, no correct symmetry can be obtained due to the error in manufacturing the beams 82a-82d, and the vibration in the X-axis direction escapes in the Y-axis direction, and the lower tip of the vibrating weight 83 effects the elliptic motion with the axis having a certain declination relative to the line X2xe2x80x94X2 as the major axis as illustrated by a broken line in FIG. 18. Thus, an offset voltage is generated in the piezoelectric elements 84b, 84d in a stationary condition, and the angular velocity detection sensitivity and the angular velocity detection resolution are degraded.
The present invention can provide an angular velocity sensor in which the above-described conventional disadvantages are solved, the angular velocity detection sensitivity is stabilized, and the angular velocity detection resolution is improved.
An angular velocity sensor in accordance with the present invention comprises a support body, a plurality of beams individually supported by the support body, and a coupling part with which a plurality of the beams are commonly coupled, and a vibrating weight formed on the coupling part, in which the each beam comprises a wide beam part and a narrow beam part narrower than the wide beam part.
According to this structure, since a beam is constituted as a coupling part of wide beam parts and narrow beam parts, the internal stress generated in manufacturing the beams, the coupling parts and the support body can be absorbed by the narrow beam parts. Therefore, the angular velocity sensor can be stably operated in a condition where no stress is present, and the detection sensitivity can be stable.
When the vibrating weight vibrates in the extending direction of the beam, or the direction to divide a space between beams into two by exciting the beam, and the narrow beam parts are between the wide beam parts and the coupling part, the vibration of the vibrating weight is absorbed by the narrow beam parts of the beam in the direction where the Coriolis force is generated, and not propagated to the wide beam parts of the beam, and thus, the offset or the noise caused by the escape of the excitation vibration contained in the detected signal to detect the Coriolis force can be reduced. The detection resolution of the angular velocity sensor can be improved thereby. In detecting the Coriolis force, the narrow beam parts of the beam to excitation-vibrate the vibrating weight are deformed, and do not suppress the vibration of the beam to detect the Coriolis force, and the detection sensitivity of the Coriolis force can be improved. When the narrow beam parts are between the wide beam parts and the support body, the coupling body of the wide beam parts with the coupling part is detached from the support body, the vibration energy of the vibrating weight is accumulated in the coupling body, and the coupling body can vibrate in a condition of high mechanical Q, and thus, the detection output by the Coriolis force can be increased.
The narrow beam part of the each beam may be coupled with the coupling part, and the wide beam part may be coupled with the support body.
According to this structure, the internal stress in the beam part is absorbed between the wide beam parts and the vibrating weight. Thus, the excitation vibration by the wide beam parts of the specified beam can be effected without suppression by other beams, and when the Coriolis force is applied to the vibrating weight, the vibration of the beam to detect the Coriolis force can be effected without suppression of the beam to be excitation-vibrated. The vibration in the exciting direction and the vibration in the direction where the Coriolis force is generated are performed without interference with each other in each beam.
Alternatively, the narrow beam part of the each beam is coupled with the support body, and the wide beam part is coupled with the coupling part.
According to this structure, the internal stress in the beam part is absorbed between the wide beam parts and the support body. As a result, the coupling body of the wide beam parts with the coupling part can vibrate like a free vibrating body with the narrow beam parts as end parts. Thus, the coupling body can vibrate in a condition where high mechanical Q is maintained by minimizing the escape of the vibration energy from the coupling body to the support body, and the detection sensitivity of the angular velocity sensor can be improved.
When the specified beam is excited, all beams vibrate together with the vibration of the vibrating weight, and the stress along with the vibration is absorbed by the narrow beam parts, and the excitation vibration is little suppressed by the support body. This also is true when the Coriolis force is applied to the vibrating weight.
Preferably, at least one piezoelectric element is formed on a wide beam part of at least one beam.
According to this structure, the angular velocity sensor can be stably operated in a condition where no initial stress is present, and the sensitivity of the angular velocity can be stable by absorbing the initial stress attributable to the piezoelectric elements formed on the wide beam parts. In particular, in the wide beam parts on which the piezoelectric elements are formed, the hysteresis characteristic of the excitation amplitude to the excitation frequency is reduced, and the unstable excitation vibration of the beam due to this hysteresis characteristic can be reduced by the narrow beam parts.
Further, even when the initial stress in the wide beam parts on which the piezoelectric element is formed is largely changed due to the change in temperature, its initial stress is absorbed by the narrow beam parts, the change in the resonance frequency of the exciting beam and the change in the resonance frequency of the beam to detect the Coriolis force can be reduced, and the temperature dependency of the detection sensitivity is improved to stabilize the detection sensitivity.
According to another aspect, the angular velocity sensor comprises a support body, four beams individually supported by the support body, a coupling part with which the four beams are coupled in an orthogonal manner, and a vibrating weight formed on the coupling part in which the each beam comprises a wide beam part and a narrow beam part, one end of the wide beam part is coupled with the support body, the narrow beam part is coupled between the wide beam part and the coupling part, and at least one piezoelectric element is formed on each wide beam part.
According to this structure, the piezoelectric element is formed on each wide beam part to separate the piezoelectric element to excite the beam from the piezoelectric element to detect the Coriolis force. In this condition, unnecessary vibration to be applied to the piezoelectric element of the detecting beam can be reduced by the action of the narrow beam parts.
According to still another aspect, the angular velocity sensor comprises a support body, four beams individually supported by the support body, a coupling part with which the four beams are coupled in an orthogonal manner, and a vibrating weight formed on the coupling part, in which the each beam comprises a wide beam part and a narrow beam part, one end of the wide beam part is coupled with the coupling part to form a cruciform coupling body, the narrow beam part is coupled between the wide beam part and the support body, and at least one piezoelectric element is formed on each wide beam part.
According to this structure, the narrow beam parts are provided between the cruciform coupling body and the support body, the suppression of the excitation vibration of the coupling body by the support body is reduced, and the coupling body can be efficiently vibrated by the piezoelectric element.
In the angular velocity sensor according to any aspect, a narrow beam part is preferably a beam forming body in which the torsion, deflection elongation and compression of the beam can be coped with by the constitution of the beam forming body. Thus, the internal stress and the initial stress in the beam can be absorbed, and the interference with other beams can be minimized, and more specifically, the mechanical coupling of the wide beam parts with the coupling part or the support body is weakened, and the vibration of one wide beam part is prevented from generating the stress in other wide beam parts.
Each narrow beam part preferably comprises at least one beam forming body of a linear shape, a Y-shape, a T-shape or a coupled linear shape with a ring which is freely deformed, and effective in reducing the transmission of unnecessary forces and in transmitting necessary forces.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.